Silicon carbide nanofiber dispersion within a silicon oxycarbide glassy ceramic was achieved through a combination of a fast solgel procedure for in situ ceramic matrix synthesis and nanofiber conversion from sacrificial multiwalled carbon nanotube templates. Nanotubes were dispersed using both surfactant adsorption and a covalent sidewall modification scheme with gel-grafting capabilities. The combination of high temperature processing and silicon monoxide precursor concentrations allowed substantial carbothermal reduction of the nanotube templates, yielding silicon carbide nanofibers. The resulting nanocomposites were examined for density, Vickers microhardness, Young's modulus, and fracture toughness. The surfactant-assisted route inhibited ceramic densification, offering virtually no mechanical property enhancement. In contrast, the covalently functionalized nanotube templates at 0.8 wt% loading enhanced tensile modulus of 77% while simultaneously maintaining both Vickers microhardness and fracture toughness. These results indicate strong interfacial adhesion between the nanofiber surface and host matrix despite the abrupt chemical changes experienced during the high temperature processing.
Abstract:
A solutionizing heat treatment of 2014 aluminum alloy reinforced with 0.15 volume fraction of alumina particles (VFAP) results in deformation-induced precipitation during rolling and tensile deformation, with 0.10 VFAP, at room temperature. The extent of precipitation increases with increase in time and/or temperature of solutionizing. An attempt has been made to identify the various types of precipitates in the samples deformed to a given strain and in fractured conditions. The work-hardening curves and tensile properties of the composites have been shown to be dependent on the time and temperature combination of the solutionizing process.
Abstract:
A study has been conducted to determine the cyclic and static thermal stability of Ti-44Al-11Nb alloy in air in a range of temperature from 900 to 1000 °C. Weight gain method shows that the static oxidation rates are higher than cyclic oxidation rates under identical experimental conditions at a given temperature. The oxide layer at the surface penetrates the base metal in a direction parallel to the lamellae of the two phases, α2 and γ, confirmed by scanning electron micrographs. The thermal stability of up to 168 hours indicates that there are phase transformations taking placed affecting the microstructures in such a way that large amount of dislocation activities are involved.
Abstract:
This work presents a process optimization study for processing vapor grown carbon fibers and single walled carbon nanotubes into Si3N4 with the goal of developing advanced structural ceramic materials. Solid composite specimens were fabricated using a freeform fabrication technique called robocasting that uses high solids loading aqueous suspensions to fabricate near-net-shape-ceramic composite parts. Colloidal processing methods were used to manipulate the charging behavior between carbon nanofibers and silicon nitride particle surfaces in order to develop forty-five percent solids loading suspensions with a pseudoplastic theology that borders on dilatancy and is suitable for robocasting solid parts. Dispersion of nanofibers within each composite system was identified as was the starting dispersion of the nanofibers in the slurry.
Abstract:
The cyclic and static oxidation behavior in the processing of Ti-44Al-11Nb alloy has been studied at 900, 950, and 1000 °C in air for a period of up to 168 hours (or a week). A recreation of diffusion paths for the oxide to penetrate into the base metal during every cycle results in the lower oxidation rates during cyclic mode compared to static mode. An extended period of heating time allows for the coarsening of the structure for both forms of alloys: polycrystalline and directionally solidified crystals. Coarsening and phase transformation in the heating period involves large amounts of dislocation activities in the lamellar structure as well as at the interface of lamellar structure and islands of γ.
